So when infrared EMR hits a substance, it oscillates electrons right? or does it oscillate entire molecules?..

Question

or does it do a little of both? Please help me to understand. Also, If I understand correctly, Emr's oscillating of particles is an example of resonance, which the scientists compare to pushing a friend on a swing.

Here is what I don't understand about the analogy: with a friend on a swing, as amplitude increases, the frequency actually decreases. But with particles being oscillated by the sun's radiant heat, does their amplitude increase similarly? And if so, isn't amplitude achievable limited by the EMR's constant f? Thanks all.

Answer 1

No , infrared is concerned ONLY by vibrations of a bound between two atoms . It is not the entire molecules, nor the electronic motions which are involved , but vibrations between atoms in a bound . amplitude concerns the number of molecules which are affectes

Answer 2

There are some things in which I don't agree with maussy:

The analogy with friend on a swing is not good - better analogy is a ball on a spring - because then the amplitude doesn't affect the frequency, just the mass of ball and the strength of spring is considered, and that is the case of the IR absorption - the frequency depends on the mass of atoms involved in the motion, and the strength of the bonds. It's not true, that only two atoms can be oscillated - this is the stretching oscillation, but also deformation oscillations exist, and those are concerning the whole molecule. The one motion has one frequency, that it will absorb - making uniform amplitude of the motion - this can be proved by quantum mechanics. If you irradiate it with double frequency (first harmonic), there will be the same vibrations, but with higher amplitude. After having really high frequency of radiation (and thus high energy), the bonds could be stretched so much, that they even get broken. (atoms get apart too much)